The 18-year-old finished joint top scorer in the League of Ireland First Division in 2012, helping Waterford to gain promotion as runners-up.
Maguire, who scored 13 goals in 24 appearances, told West Ham TV: "In summer 2011 the Waterford manager brought me in for a couple of training sessions and gave me a chance in the first team and I took it. Here I am now at West Ham and it's a dream come true really."
He added: "People tend to say my best trait is my movement so hopefully I can show that here. I haven't been told yet which squad I will be in but I presume I'll be going into the reserves so I'll try to work my way up like I did at Waterford."